1. Field of the Invention
The present invention relates to a grating interferometry-type displacement measurement device.
2. Description of the Related Art
Heretofore, a displacement measurement device (encoder) such as a rotary encoder or linear encoder has been widely used to measure displacement amounts such as the position information, movement amount, rotation amount, and so forth of a physical object. A grating interferometry-type encoder is one of these serving as an encoder to measure displacement amount with a high degree of precision. A grating interferometry-type encoder is a device that inputs coherent light to a scale where is formed a grating pattern (i.e. diffraction grating), interference is caused with diffracted light which has been diffracted by a grating pattern, and displacement amount of the scale is obtained based on the phase changes of the obtained interfered light.
Japanese Patent Laid-Open No. 2007-292735 discloses a case of applying a grating interferometry-type displacement measurement device to an exposure apparatus that transfers a desired pattern onto a substrate. The displacement measurement device measures the displacement of a reference frame of a wafer stage supporting a substrate. Diffraction grating is applied to the reference frame, and displacement of the wafer stage as to the reference frame is measured by the encoder attached to the corresponding portion on the wafer stage. According to Japanese Patent Laid-Open No. 2007-292735, upon dividing a radiation beam into positive 1st-order diffracted light and negative 1st-order diffracted light, for example, using a first diffraction grating, the light is converted into linearly polarized light wherein the polarization directions are in mutually orthogonal directions, by a linear polarizer disposed in each light path. The positive and negative 1st-order diffracted light is input into a second diffraction grating, the diffracted light is superpositioned to cause interference, and the displacement of the second diffraction grating as to the first diffraction grating is measured from the change in phase difference of the positive 1st-order diffracted light and negative 1st-order diffracted light. According to Japanese Patent Laid-Open No. 2007-292735, a linear polarizer is disposed so that the polarization of the linear polarized light obtained via the linear polarizer so that the polarization directions of the linearly polarized light are mutually orthogonal, whereby positive 1st-order diffracted light and negative 1st-order diffracted light can be distinguished.
With a grating interferometry-type encoder, how finely the phase changes can be measured becomes important in high-precision measurements. In order to measure phase changes with high precision, using a minute scale with small grating pitches of the diffraction grating is important. As a manufacturing method of a minute scale, a laser drawing method to draw a grating pattern using a laser beam (Japanese Patent No. 3,435,601) and an etching method to selectively remove layers by etching and form an uneven stepped form are widely used.
However, in manufacturing a scale with a laser drawing method, beam diameters can change on the process face, resulting from output changes of a semiconductor laser or optical systems of a drawing device, so creating a grating pattern having even line widths and thicknesses can be difficult. Also, with an etching method, creating a grating pattern having even line widths and thicknesses can be difficult, due to the influence from etching scattering on the wall portion of the grating pattern side or the unevenness of the thickness of a metallic layer or the like. Accordingly, manufacturing error such as irregularities in the line width of the grating pattern, varying thicknesses, and so forth, occur.
Therefore, in the case that the line width and thickness of the grating pattern is not even, the phase difference amount at the time of scale diffraction changed according to the measurement position. Therefore, measurement error of varying magnitudes can occur depending on the measurement position on the scale, and accurately performing displacement measurement of an object becomes difficult. Also, measurement error can occur of changes according to measurement position on the scale, not only from changes to the alternating-current components and changes to the direct-current components of the reference signal waveforms, but also from changes to phase difference of superpositioned diffracted light. Also, with a method in which obtained interference signals are corrected as with Japanese Patent Laid-Open No. 2008-185474, measurement error due to manufacturing error in the scale cannot be sufficiently corrected, so highly precise measurement is difficult.